(1) Field of the Invention
This invention relates generally to bismaleimide resins with good processibility, to the polymers made from the resins which have thermal stability and toughness, and to the production of such resins Application areas include composites, laminates, printed wiring boards, and pultrusion and resin transfer molding for the aerospace, automotive, and electronics industries.
(2) Description of the Related Art
Aromatic bismaleimides are widely used thermosetting resins having properties especially suitable for high temperature, high performance applications where they are employed as matrix resins and binders for composite molded components and laminates.
Typically, bismaleimide resin systems possess desirable thermo-oxidative and tensile properties, but poor processing properties. The poor processing properties of the resins occur because they begin to polymerize at temperatures which are at or only slightly above their melting points, making it very difficult to obtain homogeneous resin melts prior to resin gelation. This inhomogeneity is especially deleterious in the fabrication of composites or large molded parts because, upon heating, the outer portions of these fabricated pieces reach the curing temperature of the resin before the center portions of the pieces are completely melted and if the mixture is inhomogeneous, cracks and other defects are generated when final molding pressures are applied, e.g., due to high concentrations of trapped gases.
Conventional bismaleimide systems use R bridging groups such as CH.sub.2, O, S, SO.sub.2, and C(CH.sub.3).sub.2. Recently, various structural modifications in the aromatic bismaleimides have been attempted in order to improve the processibility of the resins while maintaining the desirable end use properties such as thermo-oxidative stability, mechanical strength, and solvent resistance possessed by the conventional systems. Modifications have been made to the general aromatic bismaleimide structure (I): EQU Im--Ar--R--Ar--Im (I)
where Ar is generally a phenyl or substituted phenyl ring, Im a maleimide group, and R a bridging group between the two Ar groups, by changing the structure of R, the structure of Ar, and the configuration of the attachment of R and Im to the two Ar groups.
Adams et al. ("Adams"), in U.S. Pat. Nos. 4,464,520 and 4,564,683, produced a resin with aliphatic R bridging groups S--(CH.sub.2).sub.n --S, O--(CH.sub.2).sub.n --O, and Se--(CH.sub.2).sub.n --Se (n=1-3) attached in an o,o'-configuration. However, unlike previously synthesized o,o'-resins, this resin had a "broad processing window," i.e., a broad temperature range in degrees centigrade between the peak melting temperature and the onset of polymerization as observed by differential scanning calorimetry (DSC) analysis.